Knowt
CIE AS Biology 4.3: Cell Transport Methods
Diffusion: the net movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient
Facilitated diffusion: diffusion of polar molecules and ions using proteins
Osmosis: the net movement of water molecules from an area of high water potential to an area of low water potential down a water potential gradient
Active transport: the net movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration (ATP) against a concentration gradient
Diffusion
Molecules that can move across the phospholipid bilayer by diffusion are:
small, uncharged, non-polar molecules
water
hydrophobic molecules
In the case of non-polar and hydrophobic molecules, due to their nature, they are able to pass through the hydrophobic core of the phospholipid bilayer. In the case of water and small molecules, the partial permeability of the cell membrane allows them to pass due to their size.
Diffusion occurs to form an equilibrium where the substance concentration inside the cell is equal to the substance concentration outside the cell.
Facilitated Diffusion
Molecules that move across the phospholipid bilayer by facilitated diffusion are:
large, polar molecules
ions
Due to their hydrophilic nature, they cannot pass through the hydrophobic core of the phospholipid bilayer. Instead, they need to use proteins (channel and carrier proteins) to enter a cell.
Channel proteins - water-filled pores, fixed shape; passive transport
‘gated‘ - remains closed on both ends
molecules pass freely when open
Carrier proteins - specific to one type of molecule or ion, can change shape; active and passive transport
open on one side, closed on the other side
the molecule binds specifically to the protein
Osmosis
Only water moves by osmosis.
Water potential is the term to describe the tendency of water to move in and out of a solution.
Εquation for water potential
ψ = ψs + ψp
water potential = solute potential + pressure potential
where ψs (solute potential) is the reduction in water due to the presence of solutes and ψp (pressure potential) is the hydrostatic pressure to which water is subjected. ψs is always negative and ψp is always positive.
A solution with solutes will always have a negative water potential as pure water has a water potential of 0 kPa at atmospheric pressure.
The closer the water potential value is to 0, the higher the water potential - the more dilute the solution is.
In plant cells, the process of water moving into a cell by osmosis makes the cell turgid. The process of moving water out of a cell by osmosis plasmolyses the cell.
Active Transport
Molecules that move across the phospholipid bilayer by active transport are
large molecules
ions
sugar
inorganic ions
In the case of active transport, these molecules are important for the reabsorption of useful molecules needed for cell function. They are transported by carrier proteins and use ATP to move against the concentration gradient.
Bulk Transport
Larger molecules may need to be transported in a different way from active transport. This is known as cytosis. Molecules that move across the phospholipid bilayer by cytosis are:
proteins
polysaccharides
parts of cells
whole cells e.g bacteria
There are two types of cytosis: endocytosis and exocytosis
Endocytosis - engulfing of materials into a cell using an endocytotic vacuole
pinocytosis - uptake of fluids
phagocytosis - uptake of solids; phagocytes are specialised cells; form phagocytotic vacuoles
Exocytosis - secretion of materials out of a cell using a secretory vesicle
substances such as enzymes, hormones, or cell wall building materials are secreted
secretory vesicles are made from the Golgi body
secretory vesicles fuse with the cell membrane and release their contents
Factors that Affect the Rate of Diffusion
‘steepness‘ of the concentration gradient
‘steepness‘ refers to the difference in concentration of a substance on two sides of the membrane
the greater the difference in concentration, the more molecules will randomly move across the membrane
temperature
higher temperatures give molecules and ions more kinetic energy → move faster
surface area
when there is a greater surface area, diffusion occurs at a higher rate
this can be increased by folding e.g microvilli, cristae in mitochondria
a high surface area to volume ratio means substances can diffuse more easily than a cell with a low surface area to volume ratio
properties of ions and molecules
smaller molecules will diffuse more quickly than a larger molecule
non-polar molecules can diffuse directly across the hydrophobic core of the phospholipid bilayer
CIE AS Biology 4.3: Cell Transport Methods
Diffusion: the net movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient
Facilitated diffusion: diffusion of polar molecules and ions using proteins
Osmosis: the net movement of water molecules from an area of high water potential to an area of low water potential down a water potential gradient
Active transport: the net movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration (ATP) against a concentration gradient
Diffusion
Molecules that can move across the phospholipid bilayer by diffusion are:
small, uncharged, non-polar molecules
water
hydrophobic molecules
In the case of non-polar and hydrophobic molecules, due to their nature, they are able to pass through the hydrophobic core of the phospholipid bilayer. In the case of water and small molecules, the partial permeability of the cell membrane allows them to pass due to their size.
Diffusion occurs to form an equilibrium where the substance concentration inside the cell is equal to the substance concentration outside the cell.
Facilitated Diffusion
Molecules that move across the phospholipid bilayer by facilitated diffusion are:
large, polar molecules
ions
Due to their hydrophilic nature, they cannot pass through the hydrophobic core of the phospholipid bilayer. Instead, they need to use proteins (channel and carrier proteins) to enter a cell.
Channel proteins - water-filled pores, fixed shape; passive transport
‘gated‘ - remains closed on both ends
molecules pass freely when open
Carrier proteins - specific to one type of molecule or ion, can change shape; active and passive transport
open on one side, closed on the other side
the molecule binds specifically to the protein
Osmosis
Only water moves by osmosis.
Water potential is the term to describe the tendency of water to move in and out of a solution.
Εquation for water potential
ψ = ψs + ψp
water potential = solute potential + pressure potential
where ψs (solute potential) is the reduction in water due to the presence of solutes and ψp (pressure potential) is the hydrostatic pressure to which water is subjected. ψs is always negative and ψp is always positive.
A solution with solutes will always have a negative water potential as pure water has a water potential of 0 kPa at atmospheric pressure.
The closer the water potential value is to 0, the higher the water potential - the more dilute the solution is.
In plant cells, the process of water moving into a cell by osmosis makes the cell turgid. The process of moving water out of a cell by osmosis plasmolyses the cell.
Active Transport
Molecules that move across the phospholipid bilayer by active transport are
large molecules
ions
sugar
inorganic ions
In the case of active transport, these molecules are important for the reabsorption of useful molecules needed for cell function. They are transported by carrier proteins and use ATP to move against the concentration gradient.
Bulk Transport
Larger molecules may need to be transported in a different way from active transport. This is known as cytosis. Molecules that move across the phospholipid bilayer by cytosis are:
proteins
polysaccharides
parts of cells
whole cells e.g bacteria
There are two types of cytosis: endocytosis and exocytosis
Endocytosis - engulfing of materials into a cell using an endocytotic vacuole
pinocytosis - uptake of fluids
phagocytosis - uptake of solids; phagocytes are specialised cells; form phagocytotic vacuoles
Exocytosis - secretion of materials out of a cell using a secretory vesicle
substances such as enzymes, hormones, or cell wall building materials are secreted
secretory vesicles are made from the Golgi body
secretory vesicles fuse with the cell membrane and release their contents
Factors that Affect the Rate of Diffusion
‘steepness‘ of the concentration gradient
‘steepness‘ refers to the difference in concentration of a substance on two sides of the membrane
the greater the difference in concentration, the more molecules will randomly move across the membrane
temperature
higher temperatures give molecules and ions more kinetic energy → move faster
surface area
when there is a greater surface area, diffusion occurs at a higher rate
this can be increased by folding e.g microvilli, cristae in mitochondria
a high surface area to volume ratio means substances can diffuse more easily than a cell with a low surface area to volume ratio
properties of ions and molecules
smaller molecules will diffuse more quickly than a larger molecule
non-polar molecules can diffuse directly across the hydrophobic core of the phospholipid bilayer
